What substance has specific gravity less than one?

The unlikely but true unswer is: liquid water at any temperature, except at exactly 4 degrees Celsius (normal atmospheric pressure), and, of course, water ice. Other common examples of solid substances with a density lower than 1 gram per cubic centimeter: cork, styrofoam, most wood. As common liquids, you have (besides the amazing water!) gasoline, ethanol, CH3CH2OH, a common alcohol. All gases at normal Pressure and Temperature.